Rock coring method



Patented June 6, 1950 ROCK oonINGME'rHon Y I William S. Walls, Bartlesville, Okla assignor to Phillips Petroleum Delaware mpany', a corporation of No Drawing. Application July 18, I944,

Serial No. 545,54;9

Claims.

This invention relates to'a method for reducing the contamination of cores from water drilling fluid or water entering the well bore during cable tool coring operations.

The main object of this invention is to provide a method for reducing the free water content of the drill cuttings present in the well bore during cable tool coring operations when usin oil as a drilling fluid to such an extent that the drill cuttings will be too dry to stick together to form mud or that any mud formed by these cuttings will be too dry to give up water to the formation or to the core biscuits to cause contamination of the interstitial water originally present in the formation.

In addition several more detailed objects are successfully secured by means of the method therein disclosed.

This invention resides substantially in the steps and series of steps all as hereinafter set forth and as defined in the appended claims.

Most oil and gas bearing reservoir rocks contain some water which together with oil and gas occupy the pore spaces comprising the oil and/or gas reservoir. frequently erroneously referred to as connate water, is present in most oil and gas bearing reservoirs, but the amount of the interstitial water and the percentage of saturation ofthe pore spaces by interstitial water varies widely in different oil and gas bearing reservoirs. Most oil productive sandstone reservoirs have interstitial water saturations ranging from about to 50 percent of the pore space. some oil productive reservoirs are known, however, which have in terstitial water saturations as low as 1 to 2 percent of the pore space, while others have very high water saturations in'excess of 60 per cent. Since the interstitial water occupies a portion of the effective pore space available in a given reservoir rock, a knowledge ofthe interstitial water saturation is necessary in order to determine the net effective pore space which is available for storage of oil and/or gas.

In considering secondary recovery operations from partially depleted oil reservoirs a' knowledge of the interstitial water saturation of the reservoir rock is helpful in determining the oil reserves recoverableby application of secondary recovery methods.

Cores of oil and gas bearing formations are frequently obtained by means of the rotary method using circulation of water, chemically treated mud, crude oil, or oil-base drilling fluids to remove drill cuttings .fromth'e wellbore'Qor by This interstitial formation water, 2

the cable tool method with a Baker core barrel usingwater, mud, or crude oil as a drilling fluid to keep the drill cuttings in suspension so that they can be bailed from the well bore by means of a sand pump or conventional bailer. In obtaining cores by either method it is usually found that the cores have suffered varying degrees of contamination during the coring operations. Tracers, such as dextrose, have been added to the drilling fluid in an effort to provide a means for determining the amount of contamination of interstitial water by drilling water; but these methods are not entirel reliable due to mixing of these waters and changes resulting from flushing of the formation ahead of the drill bit or expulsion of a portion' of the combined water due to gas expansion resulting from, reduction of pressure in the core while bringing it to the surface.

The use of Water-free oil or oil-base drilling fluid has enabled rotary cores containing nearly uncontaminated interstitial water saturations to be obtained from sections of formation having'low interstitial water saturation, but sections of formation having high interstitial water may lose a. portion of this water due to the flushing of the core by the oil drilling fluid. Rotary coring operations in partially depleted reservoirs are frequently not feasible due to inability to maintain circulation of drillin fluid due to loss of drilling fluid to the formation because of the high hydrostatic pressure of the column of drilling fluid. I

In the coring of partiall depleted oil or gas reservoirs having rather high permeability cable tool coring methods are usually employed since only a small amount of drilling fluid is required for coring operations, and the low hydrostatic pressure due to the short column of drilling fluid causes relatively small losses of drilling fluid to the formation. w h

In attempting to secure cable tool cores having uncontaminated interstitial water, the well is usually drilled to the top of the oil and/or gas by the drilling fluid. Since the interstitial water content of consolidated oil or gas producing sandstone formations is usually within the range of from 2 to 5 per cent by weight of the dry sand cuttings, these cuttings usually are not wet enough to stick together or form mud unless some additional water has come "into contact with them. When the drill cuttings contain about 15 per cent of water they tend to stick together to form a stifi mud, and the mud usually becomes thinner as additional amounts of water are added. When the drill cuttings containing very small amounts of water are mixed together with relatively large amounts of oil, the drill cuttings do not tend to stick together, but are rather easily maintained in the form of individual grains or pieces. Agitation of these relatively dry drill cuttings in oil tends to keep them suspended in the oil phase and facilitates drilling or coring operations by enabling the drill bit to come in more direct contact with the consolidated formation.

In cable tool coring operations conducted in the Burbank Field, Oklahoma, using water-free Burbank crude oil, it was found that even when no water could be detected in the oil phase of the drilling fluid that the drill cuttings recovered in the core barrel or by means of a sand pump, contained from 15 to 30 grams of water per 100 grams of dry sand cuttings and that the core biscuits recovered under these conditions were rather badly contaminated by water from the drilling fluid. In this case the contaminating water was the result of rotary drilling fluid, which had penetrated the top of the Burbank sand below the casing seat prior to the setting of the casing, and which returned to the well bore at a slow rate during subsequent cable tool coring operations. While the rate at which this water returned to the well bore was very slow, only about two to four quarts of water per foot of formation cored, it was taken up by the hydrophilic drill cuttings and formed a mud and water phase, which tended to remain at the bottom of the well bore and in effect served to nullify the benefits obtainable by the use of oil as the drilling fluid.

Since to avoid contamination of the original water content of the core it is necessary to keep the water content of the drill cuttings below certain maximum values; corrective measures must enable the oil phase to retain larger amounts of water and/ or aid in enabling the drill cuttings to hold more water.

The oil phase may be made to retain more water by the addition of an oil soluble emulsifying agent which will cause the water to become dispersed in the oil phase to form a water-in-oil type of emulsion. There are many suitable emulsifying agents which may be used for this purpose, but Mulsor, an organic chemical compound has been found to have very desirable properties as an additive to crude oil for this purpose, and especially for use as a cable tool drilling fluid. The addition of Mulsor, which is miscible with oil in all proportions, to the crude oil drilling fluid results in the emulsification: of the water in the oil and also greatly aids in facilitating the suspension of the drill cuttings in the oil drilling fluid by agitation.

The emulsifying or dispersing agent referred to by the trade name Mulsor is predominately a diethylene glycol mono ester of decylic acid. This material is a neutral, amber colored liquid of specific gravity of 0.973 and is miscible in all proportions with crude oil or petroleum fractions.

In accordance with this invention it is ossible, however, to still further improve the character of the oil drilling fluid with respect to avoiding contamination of the original water content of the formation, and hence all cores produced therefrom, by the addition of a drying material in suitable form to enable it to mix with the drill cuttings and maintain these drill cuttings in a suflicient degree of dryness to avoid any significant contamination of the original water content of the core. Any highly water absorbent material in granulated or chip form is suitable for the purpose but bauxite and bentonite, individually or in mixture in any proportions, have been found suitable for accomplishing this purpose. The chemicals or drying agents are preferably added to the drilling fluid in granulated or chip form so as to enable them to pass through the oil phase and to reach the bottom of the Well bore where they may take up Water to avoid the formation of a mud phase, or if mud has been formed, to dry out or condition the mud to such an extent that it will not cause contamination of the original Water content of the core.

It is pointed out that the addition of bentonitic materials to the oil drilling fluid for cable tool coring operations as disclosed herein is quite different in nature and intent from the conventional use of bentonitic materials in drilling muds. In conventional practice the bentonitic materials are added to a drilling mud to aid in conditioning this mud to give it viscosity and gel properties desired for aiding in the suspension of drill cuttings, and for other purposes. Such practice deals entirely with the presence of a mud phase, whereas in practicing the invention herein disclosed best results are obtained by maintaining the water content of the drill cuttings in a range of approximately 5 to 10 grams of water per grams of dry cuttings, under which conditions the drill cuttings are readily dispersed in the oil phase by agitation and no mud phase tends to segregate at the bottom of the well bore. It has been found that the drill cuttings usually become sticky when the water content is about 15 grams of water per 100 grams of dry cuttings and a slightly increased water content results in the segregation of a mud phase. Mulsor has been found to greatly aid in facilitating the suspension of the drill cuttings in the oil phase, especially when the water content of these cuttings is in the range of 5 to 15 grams of water per 100 grams of dry cuttings. Cores practically uncontaminated by water from drilling fluid have been obtained when the water content of the drill cuttings was in the range of 5 to 10 grams of water per 100 grams of dry cuttings. Under such conditions the drill cuttings are readily dispersed in the oil phase by agitation of the drill bit and permitted satisfactory core operations to be carried out.

In practicing this invention it has been found desirable to use the Mulsor, or its equivalent, in a proportion of about one gallon of Mulsor to from 25 to 40 gallons of crude oil, although beneficial results may be obtained from the use of greater or smaller concentrations of Mulsor or other emulsifying agents.

The drying agent should be used in a granulated or chip formation, preferably in the particle size of to A inch in diameter, although beneficial results can be obtained with slightly smaller or larger particle sizes. Powdered bentonite such as is used in the preparation of conventional drilling mud is not suitable. for the practicing of this inventionsince it has a tendency to ball up and form sticky masses which tend to plug the core barrel and may not always pass to the bottom of the well bore where it is needed for drying the drill cuttings. When water is entering the well bore at a slow rate, such as one to two quarts of water per foot of formation cored, it is frequently found that the addition of two to three quarts of a granulated bentonite having about inch particle size, which is mixed with oil and poured. into the well bore before each run of the core barrel, will enable the drill cuttings to be maintained in a sufliciently dry condition that the core biscuit will not be contaminated from water entering the well bore. It has been found desirable to keep the bentonite content of the drill cuttings rather low, preferably in the range of 5 to per cent, since high bentonitic content drill cuttings tend to become very sticky, and to form stiff muds when the water content of the cuttings increases to 15 to 30 grams of water per 100 grams of dry cuttings. It has been found that the addition of dry granulated bauxite will increase the amount of water which can be tolerated and aid in avoiding the sticky characteristics of the high bentonitic content drill cuttings.

The above discussion relates mainly to cable tool coring operations in which only relatively small amounts of water enter the well bore. This Water may be fresh or other water from some extraneous source, or may be a small amount of formation water from the formation being cored. When large amounts of water are encountered it is necessary to utilize a larger amount of drilling fluid in the Well bore in order to maintain sufficient hydrostatic pressure to overcome the formation pressure, which is tending to cause the water to enter the well bore. This is normal practice in drilling operations and in no way limits the scope of application of the method herein disclosed.

It is to be understood that while reference has been made herein to drill cuttings from sandstone formations, that this invention may be advantageously applied to other types of formations such as limestone, dolomite and other oil or gas bearing formations.

It is to be noted that this invention is not to be confused with a prior method relating to an oil base hydratable drilling fluid prepared on the surface to desired specifications, whereas in this invention the granulated drying material is pumped into the well bore to mingle with the drill cuttings and to keep them in a sufficiently dry state so that they do not form a mud phase at the bottom of the well bore. While Mulsor aids in causing dispersion of the water in the oil phase, and also keeps the drill cuttings from becoming sticky and forming mud, its use is not absolutely essential since useful advantages of this invention as herein disclosed may be secured without the use of an emulsifying agent such as Mulsor.

This invention is of practical utility since only a very small amount of water entering the well bore has been found to cause serious contamination of the cores obtained by cable tool coring operations. The application of some procedure such as that herein disclosed is necessary, or at least highly desirable, in order to obtain cores in a satisfactory condition for determining the original interstitial formation water saturation.

From the above disclosure it will be seen that this invention provides a method of reducing the free water content of the drill cuttings present in the well bore during cable tool coring operations when using oil as a drilling fluid to such an extent that the drill cuttings will be too dry to stick together to form mud, or that any mud formed by these cuttings will be too dry to give up Water to the formation or to the core biscuits to cause contamination of the interstitial water originally present in the formation.

From the above disclosure it will be apparent to those skilled in the art that the subject matter of this invention is capable of variation not only in the proportions and kinds of chemicals and drying agents employed, but also in the procedural methods of employing them. I do not, therefore, desire to be strictly limited to the disclosure as given herein for illustrative purposes.

What is claimed is:

1. In cable tool coring operations employing a drilling fluid having an oil base the method of reducing the free water content of the drill cuttings present in the bore comprising the step of adding a quantity of at least one of the water absorbing clays from the group consisting of bentonite and bauxite in the form of small pieces to the drilling fluid in the bore, said water absorbing material being in particle sizes sufflciently large to enable it to pass through the oil phase and reach the bottom of the bore and in sufficient quantity to prevent the formation or maintenance of a mud phase in the bore.

2. In the method of claim 1, said water absorbing material being in pieces having a diameter in the range of a 2- to 4; inch.

3. In the method of claim 1 the additional step of adding an emulsifying agent to the drilling fluid.

4. In the method of claim 1 the additional step of adding an emulsifying agent to the drilling fluid such as diethylene glycol mono ester of decylic acid.

5. A method of preventing the contamination of the original water content of a core by water present in an oil drilling fluid during cable tool coring operations comprising the steps of adding a suflicient amount of a water absorbing material to the drilling fluid in the bore to prevent the drill cuttings from sticking together, and mixing said material with the drill cuttings during drilling, said water absorbing material being at least one of the water absorbing clays from the group consisting of bentonite and bauxite in particle sizes of the order of 3 2 to 4; inch in diameter.

WILLIAM S. WALLS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,316,967 Miller Apr. 20, 1943 2,356,776 Miller Aug. 29, 1944 

